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.Dd December 17, 2019
.Dt FSS 4
.Os
.Sh NAME
.Nm FSS
.Nd Fair share scheduler
.Sh DESCRIPTION
The fair share scheduler (FSS) guarantees application performance by explicitly
allocating shares of CPU resources to projects.
A share indicates a project's
entitlement to available CPU resources.
Because shares are meaningful only in
comparison with other project's shares, the absolute quantity of shares is not
important.
Any number that is in proportion with the desired CPU entitlement
can be used.
.Pp
The goals of the FSS scheduler differ from the traditional time-sharing
scheduling class (TS).
In addition to scheduling individual LWPs, the FSS
scheduler schedules projects against each other, making it impossible for any
project to acquire more CPU cycles simply by running more processes
concurrently.
.Pp
A project's entitlement is individually calculated by FSS independently for
each processor set if the project contains processes bound to them.
If a
project is running on more than one processor set, it can have different
entitlements on every set.
A project's entitlement is defined as a ratio
between the number of shares given to a project and the sum of shares of all
active projects running on the same processor set.
An active project is one
that has at least one running or runnable process.
Entitlements are recomputed
whenever any project becomes active or inactive, or whenever the number of
shares is changed.
.Pp
Processor sets represent virtual machines in the FSS scheduling class and
processes are scheduled independently in each processor set.
That is, processes
compete with each other only if they are running on the same processor set.
When a processor set is destroyed, all processes that were bound to it are
moved to the default processor set, which always exists.
Empty processor sets
(that is, sets without processors in them) have no impact on the FSS scheduler
behavior.
.Pp
If a processor set contains a mix of TS/IA and FSS processes, the fairness of
the FSS scheduling class can be compromised because these classes use the same
range of priorities.
Fairness is most significantly affected if processes
running in the TS scheduling class are CPU-intensive and are bound to
processors within the processor set.
As a result, you should avoid having
processes from TS/IA and FSS classes share the same processor set.
RT and FSS
processes use disjoint priority ranges and therefore can share processor sets.
.Pp
As projects execute, their CPU usage is accumulated over time.
The FSS
scheduler periodically decays CPU usages of every project by multiplying it
with a decay factor, ensuring that more recent CPU usage has greater weight
when taken into account for scheduling.
The FSS scheduler continually adjusts
priorities of all processes to make each project's relative CPU usage converge
with its entitlement.
.Pp
While FSS is designed to fairly allocate cycles over a long-term time period,
it is possible that projects will not receive their allocated shares worth of
CPU cycles due to uneven demand.
This makes one-shot, instantaneous analysis of
FSS performance data unreliable.
.Pp
Note that share is not the same as utilization.
A project may be allocated 50%
of the system, although on the average, it uses just 20%.
Shares serve to cap a
project's CPU usage only when there is competition from other projects running
on the same processor set.
When there is no competition, utilization may be
larger than entitlement based on shares.
Allocating a small share to a busy
project slows it down but does not prevent it from completing its work if the
system is not saturated.
.Pp
The configuration of CPU shares is managed by the name server as a property of
the
.Xr project 5
database.
In the following example, an entry in the
.Pa /etc/project
file sets the number of shares for project
.Sy x-files
to 10:
.Bd -literal -offset 2n
x-files:100::::project.cpu-shares=(privileged,10,none)
.Ed
.Pp
Projects with undefined number of shares are given one share each.
This means
that such projects are treated with equal importance.
Projects with 0 shares
only run when there are no projects with non-zero shares competing for the same
processor set.
The maximum number of shares that can be assigned to one project
is 65535.
.Pp
You can use the
.Xr prctl 1
command to determine the current share
assignment for a given project:
.Bd -literal -offset 2n
$ prctl -n project.cpu-shares -i project x-files
.Ed
.Pp
or to change the amount of shares if you have root privileges:
.Bd -literal -offset 2n
# prctl -r -n project.cpu-shares -v 5 -i project x-files
.Ed
.Pp
See the
.Xr prctl 1
man page for additional information on how to modify and
examine resource controls associated with active processes, tasks, or projects
on the system.
See
.Xr resource_controls 7
for a description of the resource
controls supported in the current release of the Solaris operating system.
.Pp
By default, project
.Sy system
(project ID 0) includes all system daemons
started by initialization scripts and has an
.Dq unlimited
amount of shares.
That
is, it is always scheduled first no matter how many shares are given to other
projects.
.Pp
The following command sets FSS as the default scheduler for the system:
.Bd -literal -offset 2n
# dispadmin -d FSS
.Ed
.Pp
This change will take effect on the next reboot.
Alternatively, you can move
processes from the time-share scheduling class (as well as the special case of
init) into the FSS class without changing your default scheduling class and
rebooting by becoming
.Sy root ,
and then using the
.Xr priocntl 1
command, as shown in the following example:
.Bd -literal -offset 2n
# priocntl -s -c FSS -i class TS
# priocntl -s -c FSS -i pid 1
.Ed
.Sh CONFIGURING SCHEDULER WITH DISPADMIN
You can use the
.Xr dispadmin 8
command to examine and tune the FSS
scheduler's time quantum value.
Time quantum is the amount of time that a
thread is allowed to run before it must relinquish the processor.
The following
example dumps the current time quantum for the fair share scheduler:
.Bd -literal -offset 2n
$ dispadmin -g -c FSS
	#
	# Fair Share Scheduler Configuration
	#
	RES=1000
	#
	# Time Quantum
	#
	QUANTUM=110
.Ed
.Pp
The value of the QUANTUM represents some fraction of a second with the
fractional value determined by the reciprocal value of RES.
With the default
value of RES = 1000, the reciprocal of 1000 is \&.001, or milliseconds.
Thus, by
default, the QUANTUM value represents the time quantum in milliseconds.
.Pp
If you change the RES value using
.Xr dispadmin 8
with the
.Fl r
option, you also change the QUANTUM value.
For example, instead of quantum of 110 with RES
of 1000, a quantum of 11 with a RES of 100 results.
The fractional unit is different while the amount of time is the same.
.Pp
You can use the
.Fl s
option to change the time quantum value.
Note that such changes are not preserved across reboot.
Please refer to the
.Xr dispadmin 8
man page for additional information.
.Sh SEE ALSO
.Xr prctl 1 ,
.Xr priocntl 1 ,
.Xr priocntl 2 ,
.Xr project 5 ,
.Xr resource_controls 7 ,
.Xr dispadmin 8 ,
.Xr psrset 8
.Pp
.%T System Administration Guide: Virtualization Using the Solaris Operating System
